The present invention relates to a control circuit for controlling the pulse width of an output pulse signal by an input signal and a negative feedback signal, and more particularly, to a circuit for controlling the pulse width of a pulse signal used for determining a current supply time to an ignition coil in an ignition system for an internal combustion engine.
There is often required a circuit for generating an output signal a which has a cycle period varied in response to change in cycle period of an input signal, but has a pulse width substantially constant regardless of any change in the pulse width of the input signal. To this end, the pulse width of the output signal is controlled to be substantially constant, by the input signal and a negative feedback signal produced in response to the output signal. The term "pulse width" of a signal as used in this specification means a time period when the signal takes a high level or a low level within one cycle period.
Since the output signal is in the form of a pulse signal, the negative feedback signal is produced by converting the output signal into a first d.c. signal relative to the pulse width thereof. In more detail, the output signal is detected in the form of a pulse current flowing through a load driven by the output signal, and the pulse current is smoothed. On the other hand, the input signal is also converted into a second d.c. signal relative to a pulse width thereof. These first and second d.c. signals are utilized to control the pulse width of the output signal.
The circuit of this type can be applied to an ignition system in an internal combustion engine for the purpose of making the ignition energy from an ignition coil substantially constant. More specifically, the pulse signal in synchronism with the revolution of the engine is employed as the input signal. Therefore, the cycle period (the frequency) of the input signal is varied in accordance with the number of revolutions of the engine. The pulse width of the input signal is also varied in accordance with the number of revolutions of the engine. The output signal is used for controlling a current supply time to the ignition coil. In other words, a time period in which the output signal takes, for example, a high level (i.e., the pulse width of the output signal) affords the current supply time to the coil. The negative feedback signal is generated in the form of a first d.c. signal in response to the current flowing through the coil. The input signal is converted into a second d.c. signal relative to the pulse width of the input signal. The first and second d.c. signals are compared with each other to control the pulse width of the output signal. As a result, the pulse width of the output signal becomes substantially constant, and the sufficient and substantially constant ignition energy is generated from the ignition coil over the entire range from a low revolution speed to a high revolution speed of the engine.
If the pulse width of the input signal is, for example, shortened or prolonged for each period, there occurs such a serious problem that it becomes impossible to obtain the output signal having a predetermined pulse width. This is because the pulse width of the output signal is controlled by the input and negative feedback signals. More specifically, when the engine is under smooth revolution and producing an input signal having a pulse width corresponding to the number of revolutions of the engine, the cycle period (or pulse width) of the input signal is gradually varied in accordance with the acceleration and deceleration of the engine. Since the time-constant for the conversion of the output signal to the negative feedback signal is set smaller than such a gradual change in cycle period of the input signal, the output signal may be controlled so as to have a substantially constant pulse width. However, the engine is not always under smooth revolution. For example, the engine is started with the aid of self-starting motors. Further, in the severe winter season, the viscosity of the engine oil is increased to impair the smooth reciprocal movement of pistons. In such cases, smooth revolution of the engine can not be obtained. Consequently, the pulse width of the input signal produced in synchronism with the engine revolution, may be randomly shortened or prolonged for each period, even when the frequency of the input signal is constant. Thus, the conversion time-constant for producing the negative feedback signal becomes larger than the change in the pulse width of the input signal, thereby generating an output signal having a pulse width small than the desired one. Such an undesired output signal is generated frequently if the pulse width of the input signal undergoes random changes continually. As previously mentioned, the current supply time to the ignition coil is determined by the pulse width of the output signal. The ignition coil is an inductive element, so that a current flowing therethrough is increased gradually in accordance with the time-constant which is determined by the inductance and resistance components of the coil. Therefore, the reduction in pulse width of the output signal causes a malfunction that the current flowing through the ignition coil does not reach the predetermined value. In other words, insufficient ignition energy is generated from the ignition coil. The insufficient ignition energy results in incomplete combustion of a gas mixture within the cylinder. If such a condition is repeated frequently, environmental pollution will be caused.